The invention relates to an electron beam tube having input resonator cavity through which a high-frequency control voltage is applied between cathode and grid of the tube, while an annular electrode connected in an electrically conducting manner to the grid of the tube is arranged opposite a metal wall of the input resonator cavity and is connected to a DC power supply lead.
Such an arrangement is described in GB-A 2 259 708. In the arrangement described in this document a ceramic material cylinder enclosing a cathode, an anode and a grid is surrounded by a coating of silicon rubber comprising ferrite particles. Self-oscillations of a resonant circuit present between the anode and the grid are prevented thereby. Such self-oscillations may also occur between the cathode and the grid and then cause interference modulation of the electron beam, and in extreme cases may even lead to destruction of the tube.
It was found that disturbing self-oscillations occur in spite of the measures taken in the known case.